1. Field of the Invention
This invention relates to the formation of a hydrophilic copolymer useful in the formation of hydrogel contact lenses and other products.
2. Description of the Prior Art
Recently there has been a great deal of effort devoted to finding new materials which would be more suitable for contact lenses than the polymethyl methacrylate customarily used.
One avenue of approach has been to search for softer and/or more flexible materials, and some of these newer type materials which are described in the patent literature include: silicone rubbers such as those described in McVannel et al., U.S. Pat. No. 3,350,216; poly(4-methyl penetane-1) which is described in Kamath, pentene-Pat. Nos. 3,551,035 and 3,619,044; and fluoropolymers such as those described in Girard, U.S. Pat. No. 3,542,461.
Another avenue of approach has been to search for materials which would have better compatibility with eye fluids. Thus, some researchers have made attempts to treat the surfaces of naturally hydrophobic materials to render them hydrophilic. Others have formed copolymers including at least one monomer which is hydrophilic or which can be treated to make it hydrophilic.
To date, the most widely accepted of the new, hydrophilic, softer contact lens materials are hydrogels formed from aqueous solutions gelled with sparingly cross-linked acrylic polymers. Wichterle et al., U.S. Pat. No. Re. 27,401, describe in general one class of these hydrogels which can be formed from 20 to 97% of an aqueous liquid gelled with a sparingly cross-linked copolymer formed from a major amount of a hydrophilic monoester of acrylic or methacrylic acid cross-linked with a minor amount of a diester of these acids. The commercial soft contact lenses of the Wichterle type appear to contain a polymer network formed from large amounts of hydroxyethyl methacrylate cross-linked with small percentages of glycol dimethacrylate.
Other hydrogels presently described in the patent literature contain certain acrylic monomers together with vinyl pyrrolidone type polymers. See, for example, Seiderman, U.S. Pat. No. 3,639,524 and Leeds, U.S. Pat. No. 3,621,079.
More recently issued patents to Stamberger disclose the use of glycidyl esters of acrylic, methacrylic or crotonic acids as substitutes for the heretofore traditional cross-linking agents, i.e., glycol diacrylates or dimethacrylates. Thus, U.S. Pat. No. 3,758,448 discloses the use of such cross-linking esters in hydrogels based upon water soluble monoesters of acrylic or methacrylic acids with a polyhydric alcohol, whereas U.S. Pat. No. 3,787,380 discloses the use of such cross-linking esters in hydrogels based upon heterocyclic N-vinyl monomers such as N-vinylpyrrolidones and alkyl acrylates or methacrylates.
Despite large amounts of research previously conducted, it is generally agreed that an ideal contact lens has not heretofore been available. This is because of the many stringent requirements of materials suitable for contact lenses. For example, the hydrated material must have sufficient optical clarity and must have structural integrity such that its shape and size do not change over prolonged periods of time under various environmental conditions which would, of course, result in a change in the visual acuity of the wearer.
In addition to these optical properties, suitable materials should have certain other mechanical and durability properties in both the hydrated and anhydrous states. Hydrated materials should be tough, resilient, resistant to compressive and flexural stresses, and should resist tearing and/or nicking. Durability can be further enhanced if the hydrated materials do not become brittle and/or friable upon drying. The anhydrous material should be machinable, such as on a lathe, and readily polishable. It may also be advantageous if the polymerization can be carried out in a mold with or without solvent thereby providing the capability of forming shaped objects without further machining operations, or by spin casting polymerization.
On the other hand, the hydrated lens material should have a soft texture and feel and the capability to transmit fluid and gases such as oxygen and carbon dioxide. These properties would greatly increase the wearing comfort and allowable wearing time of a contact lens.
It is highly desirable, of course, to have contact lenses which can be left in the eye for extended periods of time. Such extended-wear lenses depend upon an oxygen permeability which allows sufficient oxygen to be transported through the lens to provide normal corneal respiration. Oxygen permeability is known to depend upon lens thickness, and is also known to increase with increasing amounts of lens hydration.